The spine includes a series of joints known as motion segment units. Each unit represents the smallest component of the spine that exhibits a kinematic behavior characteristic of the entire spine. The motion segment unit is capable of flexion, extension, lateral bending, and translation. The components of each motion segment unit include two adjacent vertebrae, the corresponding apophyseal joints, an intervertebral disc, and connecting ligamentous tissue, with each component of the motion segment unit contributing to the mechanical stability of the joint. For example, the intervertebral discs that separate adjacent vertebrae provide stiffness that helps to restrain relative motion of the vertebrae in flexion, extension, axial rotation, and lateral bending.
When the components of a motion segment unit move out of position or become damaged due to trauma, mechanical injury or disease, severe pain and further destabilizing injury to other components of the spine may result. In a patient with degenerative disc disease (DDD), a damaged disc may provide inadequate stiffness, which may result in excessive relative vertebral motion when the spine is under a given load, causing pain and further damage to the disc. Depending upon the severity of the structural changes that occur, treatment may include fusion, discectomy, and/or a laminectomy.
Current surgical treatments often involve fusion of unstable motion segment units with removal of adjacent tissue. For numerous reasons, fusion may be an undesirable treatment option. For instance, fusion results in a permanent, rigid fixation with irreversible loss of range of motion at fused vertebral levels. In addition, loss of mobility at the fused levels causes stress to be transferred to other neighboring motion segments, which can cause or accelerate degeneration of those segments. Moreover, fusion often does not alleviate some or all of the pain.
It would thus be desirable to provide a spinal stabilization system that is sufficiently functionally dynamic to manage the load sharing characteristics of the treated spine. It would further be desirable to provide a system that would allow close-to-normal motion, mimicking the physiological response of a healthy motion segment and providing pain relief.